1.2.1 Systems Software

collection of programs that work together to give an interface between user and computer

memory must be shared fairly between multiple programs as ram not large enough to hold all. so os allocated amount of procosser time each task gets

paging, segmentation and virtual memory

memory is split up into equal sized section these can be swapped in and out of memory from secondary storage. However could seperate instructions like a loop.

splits memory into logical size division which may vary in size. Represents the structure and logical flow of program.

uses a section of seconday storage to act as ram. part of program not in use temp moved through paging. However disk thrashing can occur, this is when the computer freezes due to pages being swapped to frequently so they are spending move time moving instructions then executing them.

Are signals generated by software or hardware to indicate to the cpu that a process needs attention

based on priority on urgent stored in order of priority within an abstract data structure callled priority qeue which is in the interrupt register

1)At the end of the fde cycle processor checks content of interrupt register 2) if an iterrupt of higher prority is found the content of the memory registers are temp transfered to a stack 3)cpu then responds to interrupt by loading the correct interrupt servies routine into the ram. A flag is sent to signal that the ISR began 4) Queue checked again for interrupts, this is repeated untill all have been servises. All interrupts have higher priorty then regular programs 5) pop the frame of the stack to continue the fde cycle

function of OS to ensure all sections of programs being run (jobs) recieve a fair amount of processing time. How long , what next ??

pre-emptive non pre-emptive

Pre - jobs are activly made to stop and start by using the os non pre - once job starts left alone untill completion

Round robin multi-level feedback shortest time remaining

First come first serve shortest job first

Each job given section of processor time (slice) to execute. All jobs have equal time untill complete and removed from queue. Long jobs are ineff , doesnt take prority into account

Chronological order from joining queue

multiple queues with diffrent priorites , difficult to implicate

req processer to calc how task takes. risks processor stavation if continuious short jobs join queue

req processer to calc how task takes. risks processor stavation if continuious short jobs join queue but if a shorted job joins queue it will take priorty

1) distributed - runs across multiple devices allowing the load to be spread accross multiple computer cpus 2) Embedded - preforms a small range of specific tasks consumes less power 3) multi-tasking - multiple userrs make use of some computer must use scheduling alg risk of cpu starvation 4) real time - used in critcal time computers were takes happen in a guaranteed time frame

Basic input output system, first system to run pc relies on it for running tests before loading OS into memory

1) POST - power on self test ensures hardware are correctly connected and functional 2) checks cpu clock , memory and cpu is operational 3) tests for external memory

As OS can only be loaded into the RAM from secondary storage

Loads OS kernel into main memory